1. Technical Field of the Invention
The present invention relates to surface contamination examining device and method that examine surface contamination by a radioactive material.
2. Description of the Related Art
Patent Documents 1 to 6 disclose a device for examining surface contamination by a radioactive material in nuclear facilities.
Patent Document 1 discloses a “radiation monitor” that automatically sets an optimum contamination counting time depending on a change of background. The radiation monitor transports an object to be measured on a conveyor and determines a contamination state of the object to be measured. As shown in FIG. 1, the radiation monitor includes a background measuring unit 114 that measures a background level, a counting time deciding unit 115 that finds a contamination counting time, which puts a true counting value having no background into a contamination control level, on the basis of the measurement value by the background measuring unit 114 and a prescribed contamination control level, and a speed control unit 111 that controls the speed of the conveyor in accordance with the contamination counting time decided by the counting time deciding unit 115.
Patent Document 2 discloses a “goods transport monitor” that improves workability of contamination examination of goods to be transported. The goods transport monitor transports goods 133 to be examined into a monitor main body 120 by an automatic transport path 123, measures the dose of radiation emitted from the goods 133 to be examined by a radiation detector 129 in the monitor main body 120, and monitors presence/absence of contamination of the goods 133 to be examined on the basis of the measurement value. As shown in FIG. 2, the goods transport monitor includes a stability detecting unit 126 that is provided near to the automatic transport path 123 on a goods insertion side of the monitor main body 120, and a transport speed control unit 131 that adjusts a transport speed of the goods to be examined in accordance with the stability of the goods 133 to be examined detected by the stability detecting unit 126.
Patent Document 3 discloses “surface contamination examining device and method” that efficiently and automatically determine presence/absence of radiation contamination of an object to be measured while being moved, and accurately and efficiently measure the contamination of the object to be measured with high reliability as occasion demands. As shown in FIG. 3, the surface contamination examining device includes a radiation detecting unit 223 that measures the dose of radiation to be emitted while changing a part to be measured according to movement of an object 211 to be measured, a monitoring and determining unit 220 that monitors an upward trend of a measurement value from the radiation detecting unit 223, and a motor speed control unit 218 that, when the upward trend of the measurement value is recognized by the monitoring and determining unit 220, controls a conveyor driving motor to reduce or change a conveyor speed or to stop a conveyor. When the upward trend of the measurement value is recognized by the monitoring and determining unit 220, the conveyor speed is reduced or the conveyor is stopped to measure the dose of emission radiation from the object 211 to be measured and determine presence/absence of contamination of the object to be measured.
Patent Document 4 discloses a “radiation monitor” that promptly and reliably detects an abnormal increase of a radiation level in an initial stage, and promptly takes a countermeasure. Referring to FIG. 4, as for a data processing for detecting an increase in the radiation level, N is acquired at every predetermined time t, latest data at each time is set to N1, and a new counting time band Ts including N1 at each counting and a previous old counting time band Tb are set. An increase level counting rate n which is a difference between counting rates ns and nb is obtained, and an increase level determination value a is obtained by using a relational expression of a statistical error in the measurement of the counting rate n. The counting rate n and the determination value a are compared with each other. When the counting rate n is larger than the determination value a, a forecasting alarm is operated. When the counting rate n is smaller than the determination value a, the time band Ts is extended, the old counting time band Tb is shifted forward by the extended portion of the time band Ts. Similarly, the increase level counting rate n and the increase level determination value a are obtained and compared with each other, and a forecasting determination is performed. This processing is repeatedly performed until the time band Ts reaches a prescribed time band Tmax. The same processing is sequentially performed at a next count t after the time band Ts reaches the time band Tmax. Patent Document 5 discloses a “radiation measuring device” that, in a surveymeter, efficiently specifies a contaminated part without being captivated by background. Referring to FIG. 5, if a detection pulse 300 is output from a wave-height discriminator 314, a pulse 302 having a predetermined width T is output from a one shot multivibrator 316. If another detection pulse 300 is generated outside the width T, it is determined as background by AND circuits 318 and 320, and a detection pulse other than the background is output as a measurement result.
Patent Document 6 discloses a “detector with visual display in a surveymeter” which, when radioactive contamination is measured by a surveymeter of a type with a detector and a measuring device separated, is capable of measuring presence/absence of radioactive contamination without requiring frequent motions of eyes and without generating click sound to acoustically inform a level of contamination, and in which visual display is turned on/off when a radioactive measurement value exceeds a control reference value. Referring to FIG. 6, in a surveymeter of a type with a detector and a measuring device separated, a visual display unit having one or a plurality of illuminants 303 is set to a part, such as a rear surface of a hold portion 304 of the detector, in which a measurer visually recognizes with ease. The illuminants 303 are turned on/off in synchronization with radiation pulses input to a scintillator 301 of the detector.
[Patent Document 1] Japanese Patent Application Laid-Open No. 6-148334, “Radiation Monitor”
[Patent Document 2] Japanese Patent Application Laid-Open No. 6-64714, “Goods Transport Monitor”
[Patent Document 3] Japanese Patent Application Laid-Open No. 18-177883, “Surface Contamination Examining Device and Method”
[Patent Document 4] Japanese Patent Application Laid-Open No. 6-324150, “Radiation Monitor”
[Patent Document 5] Japanese Patent Application Laid-Open No. 10-197644, “Radiation Measuring Device”
[Patent Document 6] Japanese Patent Application Laid-Open No. 13-228256, “Detector with Visual Display in Surveymeter”
As described above, in the known devices for detecting surface contamination by a radioactive material, like Patent Document 1, the optimum contamination counting time is automatically set according to the change of background. In addition, like Patent Document 2, the examination speed of the object to be measured is adjusted to improve workability, and like Patent Document 3, the examination mode of the object to be measured varies depending on the reliability factor.
Like Patent Document 4, presence/absence of contamination is found in the initial stage on the basis of an increase rate of the counting rate from various time bands obtained measurement data.
Like Patent Document 5, background or contamination is determined according to a time interval at which a detection pulse comes.
Like Patent Document 6, the control reference value is provided, and the illuminants are turned on/off when the radioactive measurement value exceeds the control reference value.
A surveymeter used in the known examining devices has a radiation detector that detects radiation, and an arithmetic/display device that processes an output pulse from the radiation detector and displays radiation intensity (for example, displays a counting rate: cpm). However, as shown in FIG. 7, when a response approximates as a primary delay system, and a time constant indicative of the response is set to 10 seconds, even if the surveymeter is stationary at the same position and performs measurement, it takes a time, for example, strictly 60 seconds or typically 30 seconds, which is three times larger than the time constant, to reach an actual radiation intensity.
For this reason, in the known examining devices, the time constant of the surveymeter is changed to 3 seconds, and thus a detection time of presence/absence of contamination is shortened. In this case, however, the surveymeter needs to be stationary at the same position around ten seconds.
In order to detect presence/absence of contamination and to grasp the range of contamination, that is, the boundary of contamination (contamination start position and contamination end position), it is necessary to perform the examination while moving the surveymeter over the entire potential contamination range.
In this case, as shown in FIG. 8, the stationary measurement value is high in the vicinity of the contamination range. When detection is performed while the surveymeter is being moved, a detection value at the contamination start position is low due to detection delay, and a detection value at the contamination end position is high due to detection delay. Accordingly, it is impossible to decide the contamination start position and the contamination end position.
For this reason, in accurately grasping the boundary of contamination (the contamination start position and the contamination end position), it is necessary to repeatedly reciprocate or stop many times in the vicinity of the contamination. As a result, it takes a lot of time to specify a contaminated location.